1. Field of the Invention
The present invention generally relates to a multiple access apparatus and method in a mobile communication system. More particularly, the present invention relates to a multiple access apparatus and method in a mobile communication system supporting a variety of multiple access schemes.
2. Description of the Related Art
Typically, mobile communication systems provide communication service to a large number of users irrespective of time and place. The mobile communication systems provide access to users in various multiple access schemes.
Two principal types of multiple access schemes used in mobile communication systems are a non-orthogonal multiple access scheme and an orthogonal multiple access scheme. Signals sent from a plurality of Mobile Stations (MSs) are non-orthogonal in the non-orthogonal multiple access scheme, whereas the signals are orthogonal in the orthogonal multiple access scheme.
A major non-orthogonal multiple access scheme is Code Division Multiple Access (CDMA). cdma2000 and Wideband CDMA (WCDMA) adopt CDMA in which a plurality of MSs send data, sharing the same frequency at the same time. Each MS is identified by a user-specific scrambling code (scrambling sequence or Pseudo-Noise (PN) sequence). Although there is no orthogonality among the scrambling sequences of different MSs, a signal received from a particular MS can become stronger by use of a processing gain, thereby making the MS identifiable.
FIG. 1 is a block diagram of a typical CDMA transmitter.
Referring to FIG. 1, a channel encoder 101 channel-encodes an input information bit sequence according to a coding method. The channel encoder 101 can be a block encoder, a convolutional encoder, a turbo encoder, or a Low Density Parity Check (LDPC) encoder. A channel interleaver 102 interleaves the coded data according to an interleaving method. While not shown in FIG. 1, it is clear that a rate matcher including a repeater and a puncturer can reside between the channel encoder 101 and the channel interleaver 102. A modulator 103 modulates the interleaved data in Quadrature Phase Shift Keying (QPSK), 8-ary Phase Shift Keying (8PSK), 16-ary Quadrature Amplitude Modulation (16QAM), or the like. A Walsh coverer 104 Walsh-covers the modulation symbols. Typically, physical channels that an MS sends include a pilot channel, a traffic channel, a pilot control channel, etc. A different Walsh function is preset for each physical channel. Thus, the MS performs the Walsh covering using a predetermined Walsh function for a physical channel to be sent.
A gain controller 105 multiplies the output of the Walsh coverer 104 by a gain suitable for the physical channel according to a predetermined rule. The channel encoding in the channel encoder 101 to the gain control in the gain controller 105 take place independently for each physical channel. The gain-added outputs for physical channels are summed in an adder 106. A mixer 107 multiplies the sum by a user-specific scrambling sequence. A baseband filter 108 converts the scrambled signal to a final baseband signal.
FIG. 2 is a block diagram of a typical CDMA receiver.
Referring to FIG. 2, a baseband filter 201, which is a matched filter corresponding to the baseband filter 108 illustrated in FIG. 1, filters a received signal. A mixer 202 multiplies the output of the filter by a user-specific scrambling sequence and a Walsh decoverer 203 decovers the descrambled signal with a Walsh function preset for a physical channel to be demodulated. A channel equalizer 204 channel-equalizes the Walsh-decovered signal according to a predetermined method. The channel equalization can be performed in many ways, which are beyond the scope of the present invention. A demodulator 205 demodulates the channel-equalized signal according to a predetermined demodulation method such as 16QAM, 8PSK, QPSK, or the like. A channel deinterleaver 206 deinterleaves the demodulated signal and a channel decoder 207 channel-decodes the demodulated signal. Thus, the original information is finally recovered.
Principal orthogonal multiple access schemes include Orthogonal Frequency Division Multiple Access (OFDMA), Time Division Multiple Access (TDMA), and Single-Carrier Frequency Division Multiple Access (SC-FDMA). OFDMA is a multiple access scheme in which a plurality of MSs send signals on different subcarriers. In TDMA, MSs send signals at different times, and in SC-FDMA, MSs send signals in different carrier frequencies. With reference to FIGS. 3 to 8, an OFDMA transmitter and receiver and an SC-FDMA transmitter and receiver will be described below in detail.
FIG. 3 is a block diagram of a typical OFDMA transmitter.
Referring to FIG. 3, a channel encoder 301, a channel interleaver 302, a modulator 303, and a gain controller 304 operate in the same manner as their counterparts illustrated in FIG. 1 and thus their description will not be provided herein. A Serial-to-Parallel Converter (SPC) 305 converts a serial gain-controlled signal to parallel signals. A subcarrier mapper 306 maps the parallel signals to subcarriers according to a predetermined mapping method.
Compared to CDMA in which physical channels sent by one MS are differentiated by covering them with different Walsh codes, they are differentiated by sending them on different subcarriers in OFDMA. In other words, the channel encoding in the channel encoder 301 to the serial-to-parallel conversion in the SPC 305 are performed independently for each physical channel. The parallel signals are mapped to subcarriers preset for the physical channel. An Inverse Fast Fourier Transform (IFFT) processor 307 IFFT-processes the output of the subcarrier mapper 306. A Parallel-to-Serial Converter (PSC) 308 converts parallel IFFT signals to a serial signal. A Cyclic Prefix (CP) adder 309 inserts a CP in the serial signal according to a predetermined method. A baseband filter 310 converts the CP-added signal to a final baseband signal.
FIG. 4 is a block diagram of a typical OFDM receiver.
Referring to FIG. 4, a baseband filter 401, which is a matched filter corresponding to the baseband filter 310 illustrated in FIG. 3, filters a received signal. A CP remover 402 removes a CP from the output of the baseband filter 401 according to a predetermined method. An SPC 403 converts the CP-free signal to parallel signals. A Fast Fourier Transform (FFT) processor 404 FFT-processes the parallel signals. A subcarrier demapper 405 extracts subcarriers mapped to a physical channel and a channel equalizer 406 channel-equalizes the subcarriers. A PSC 407 serializes the channel-equalized signal and a demodulator 408 demodulates the serial signal according to a predetermined demodulation method such as 16QAM, 8PSK, QPSK, or the like. A channel deinterleaver 409 deinterleaves the demodulated signal according to a predetermined method and a channel decoder 410 channel-decodes the deinterleaved signal, thereby recovering the original information.
FIG. 5 is a block diagram of a typical SC-FDMA transmitter.
Referring to FIG. 5, a channel encoder 501, a channel interleaver 502, a modulator 503, a CP adder 504, a gain controller 505, and a baseband filter 506 operate in the same manner as their counterparts illustrated in FIG. 3 and thus their description will not be provided herein. The output of the baseband filter 506 is subject to a user-specific phase rotation, for signal identification in a phase rotator 507. Thus, a final baseband signal is produced. The phase rotator 507 functions to send signals to MSs in different frequencies. Before the user-specific phase rotation, the transmission signal takes the form of a low pass signal as indicated by reference numeral 511. After the phase rotation, it takes the form of a predetermined-band pass signal, as indicated by reference numeral 512.
FIG. 6 is a block diagram of a typical SC-FDM receiver.
Referring to FIG. 6, a phase derotator 601 phase-derotates a received signal, for MS identification. Before the phase derotation, the received signal takes the form of a predetermined-band pass signal, as indicated by reference numeral 611. After the phase derotation, it takes the form of a low pass signal, as indicated by reference numeral 612.
A baseband filter 602, which is a matched filter corresponding to the baseband filter 506 illustrated in FIG. 5, filters the phase-derotated signal. A CP remover 603 removes a CP from the output of the baseband filter 602 according to a predetermined method. A channel equalizer 604 channel-equalizes the CP-free signal. A demodulator 605 demodulates the channel-equalized signal according to a predetermined demodulation method such as 16QAM, 8PSK, QPSK, or the like. A channel deinterleaver 606 deinterleaves the demodulated signal according to a predetermined method and a channel decoder 607 channel-decodes the deinterleaved signal, thereby recovering the original information.
While the transmitter and the receiver illustrated in FIGS. 5 and 6 implement SC-FDMA in the time domain, they may implement SC-FDMA in the frequency domain.
FIG. 7 is a block diagram of a typical SC-FDMA transmitter that implements SC-FDMA in the frequency domain.
Referring to FIG. 7, a channel encoder 701, a channel interleaver 702, a modulator 703, and a gain controller 704 operate in the same manner as their counterparts illustrated in FIG. 1 and thus their description will not be provided herein. An SPC 705 converts a serial gain-controlled signal to parallel signals. An FFT processor 706 FFT-processes the parallel signals and a subcarrier mapper 707 maps the FFT signals to subcarriers according to a predetermined method. The subcarrier mapper 707 functions to enable a signal for the MS to occupy a predetermined frequency as indicated by reference numeral 512 in FIG. 5. An IFFT processor 708 IFFT-processes the output of the subcarrier mapper 709. A PSC 709 converts parallel IFFT signals to a serial signal. A CP adder 710 inserts a CP in the serial signal according to a predetermined method. A baseband filter 711 converts the CP-added signal to a final baseband signal.
FIG. 8 is a block diagram of a typical SC-FDMA receiver that implements SC-TDMA in the frequency domain.
Referring to FIG. 8, a baseband filter 801, which is a matched filter corresponding to the baseband filter 711 illustrated in FIG. 7, filters a received signal. A CP remover 802 removes a CP from the output of the baseband filter 801 in a predetermined method. An SPC 803 converts the CP-free signal to parallel signals. An FFT processor 804 FFT-processes the parallel signals. A subcarrier demapper 805 extracts mapped subcarriers as described with reference to FIG. 7 and a channel equalizer 806 channel-equalizes the subcarriers according to a predetermined channel equalization method. An IFFT processor 807 IFFT-processes the channel-equalized signal and a PSC 808 serializes the IFFT signals. A demodulator 809 demodulates the serial signal according to a predetermined demodulation method such as 16QAM, 8PSK, QPSK, or the like. A channel deinterleaver 810 deinterleaves the demodulated signal according to a predetermined method and a channel decoder 811 channel-decodes the deinterleaved signal, thereby recovering the original information.
The non-orthogonal multiple access scheme and the orthogonal multiple access schemes have their own advantages and weaknesses. For example, CDMA suffers from interference between signals from MSs because the signals are not orthogonal. Hence, a relatively high Signal-to-Noise Ratio (SNR) cannot be expected for a signal from a particular MS. Despite this shortcoming, CDMA facilitates scheduling in that MSs send signals, sharing the same frequency at the same time. Therefore, the non-orthogonal multiple access scheme is favorable for voice communication or frequent transmissions of real-time small packet data.
In contrast, due to orthogonality among signals from MSs, OFDMA enables a relatively high SNR for a signal from a particular MS, which makes OFDMA suitable for high-peed packet transmission. Yet, support of orthogonality requires accurate scheduling. That is, orthogonal resources used by a plurality of users, i.e. subcarriers in OFDMA, transmission time in TDMA, and frequencies in FDMA need precise centralized control. In this context, OFDMA is suitable for high-speed packet transmission, but not viable for voice communications or frequent transmissions of real-time small packet data.
As described above, the orthogonal and non-orthogonal multiple access schemes have different characteristics and advantages in different aspects. Accordingly, it will be inefficient to support all services with different properties and requirements with one multiple access scheme.